This invention relates to a process for producing semi-hard magnetic materials suited for self holding type reed switches and, more particularly, a process for producing cobalt-niobium-iron based semi-hard magnetic materials having a desired form of hysterisis loop and remanent property required for small-sized reed switches. The semi-hard magnetic material generally means a magnetic material having intermediate properties between the soft magnetic material and permanent magnetic material, i.e. hard magnetic material. The semi-hard magnetic material utilizes the remanence property like the permanent magnet and is required to have a property permitting ready rotation of domains, similar to the soft magnetic material. In general, semi-hard magnetic materials utilized in such applications should possess the following properties.
1. Both saturated magnetic flux density (Bs) and residual magnetic flux density (Br) are high.
2. The square hysteresis loop exhibits a high squareness ratio (Br/Bs) and a high "fullness factor" in the second quadrant. The term fullness factor used herein is represented by the formula: EQU .sqroot.(BH).sub.max /Br Hc
Wherein (BH).sub.max is the maximum magnetic energy product and Hc is coercive force.
3. The coercive force (Hc) is within the range of 10 and 50 Oe.
4. The plastic workability characteristic is excellent, since these materials are capable of being easily worked into any desired shape such as, for example, a fine wire rod, a sheet or a tape.
Some of the inventors of the present invention previously provided a process for producing such semi-hard magnetic materials, in which process an alloy consisting essentially by weight of 73 to 93% of cobalt, 1 to 5% of niobium and the balance being iron is worked into a semi-finished product, subjected to a final process annealing at a temperature not lower than 900.degree.C, and subsequently to a final cold-working at a reduction of area not lower than 75%. Said some inventors have also previously provided a process for producing the semi-hard magnetic materials, wherein after the final cold-working mentioned above the alloy is subjected to ageing at a temperature not higher than 950.degree.C.
Due to the recent trend toward miniaturization of reed switches it is required that the semi-hard magnetic material used in such switches possess a coercive force of not lower than 20 Oe. In order to produce semi-hard magnetic material having the desired coercive force by means of the known process, the content of niobium in the alloy should be maintained in the range of 3 to 5%. In other words, the alloy containing 1 to 3% of niobium cannot be used for miniaturized reed switches. The increase in the content of niobium brings about the formation of an intermetallic compound of niobium. It is, therefore, possible to produce a semi-hard magnetic material having moderate coercive force, i.e. not lower than 20 Oe, if the precipitated amount of the intermetallic compounds is controlled by any suitable method.
The increase in the content of niobium is, however, accompanied by a promoted hardening of the alloy resulting from both a solid solution hardening and a precipitation hardening. The alloy is, therefore, quite hard during the process of cold working so that the cold-working becomes extremely difficult. Experiments have shown that the alloy material containing more than 4% of niobium obviously tends to rupture while being drawn into a wire-rod, with the result that the desired semi-hard magnetic material can not be produced on a mass production scale.
Therefore, only if the final process annealing is followed by extremely slow cooling or if the final cold drawing is divided into a number of drawing steps each causing an extremely small reduction of area, for example approximately 2%, can the alloy be worked into the finished product, thereby ensuring the total reduction of area to the desired value. Such a slow cooling or repeated cold working at a low reduction of area should be avoided from a practical point of view because both of the processes for producing the semi-hard magnetic materials are too complicated and expensive.